In many electronic circuit applications, such as power converters, semiconductor switches (also referred to as solid state switches) are used. Semiconductor switches are, e.g., MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors), IGBTs (Insulated Gate Bipolar Transistors), JFETs (Junction Field-Effect Transistors), BJTs (Bipolar Junction Transistors) or HEMTs (High Electron-Mobility Transistors). In operation of an electronic switch, different types of failures may occur, such as a short-circuit of a load connected to the switch. In order to prevent the electronic switch from being damaged in case of a failure, at least one operation parameter of the electronic switch may be monitored and suitable measure may be taken, such as switching off the electronic switch, when the operation parameter indicates that a failure has occurred.
On device parameter that may be monitored is a load-path voltage of the electronic switch. In an IGBT or a BJT, the load-path voltage is a collector-emitter voltage (VCE), in a MOSFET or JFET, the load path voltage is a drain-source voltage (VDS). The load-path voltage is dependent on a load current through the electronic switch and increases as the load current increases when the load current is below a nominal current, that is when the electronic switch is operated in a linear region (ohmic region) of its characteristic curve. At higher load currents, that is when an IGBT is operated in the desaturation region of its characteristic curve, or when a MOSFET, a JFET or a HEMT is operated in the saturation region (pinch-off region) of its characteristic curve, the load-path voltage may dramatically increase which may cause the electronic switch to be damaged or even destroyed, unless suitable measures are taken.
For normally off devices such as, e.g., MOSFETs or IGBTs, that have a driver with a positive power supply, there are several known solutions to implement a protection circuit that monitors an operation parameter and switches off the electronic switch in case of a failure. These solutions use the positive power supply which is available anyway. These solutions, however, may not be suitable to be used for normally-on devices such as, e.g., JFETs or HEMTs. Normally-on devices have a driver with a negative power supply. Therefore, there might not be a positive power supply available or there is only a positive power supply available that does not provide a positive supply voltage that is high enough to implement the same protection circuitry as used for normally-off devices.
The cost for providing an additional power supply for the protection circuitry of normally-on devices can be very high, and deriving a positive supply voltage for the protection circuitry from a higher supply voltage of the load can be very power consuming and/or expensive.
There is therefore a need for a circuit including an electronic switch and a monitoring circuit that is less expensive, less power consuming and easy to be implemented.